1. Field of the Invention
This invention relates to key telephone system electrical circuitry. In particular the invention relates to a calling circuit in which audio signals, lamp signals and control signals are superimposed over four conductors between a telephone station set and a main control circuit.
2. Description of the Prior Art
A prior art circuit similar to the circuit shown in FIG. 1 of the drawings, used eight conductors to connect a main apparatus 1 and a telephone set 2. Over these eight conductors, the prior art calling circuit passed two types of audio signals, as well as control signals and lamp signals. In FIG. 1, box 4 represents a first audio signal source, such as a musical broadcast or a voice paging signal. Box 5 represents a second audio signal source, such as a chime or bell signal to alert the telephone station user that the station is being called. The circuit labeled 3 is a detector which detects the state of the hook switch 11 of the telephone set and is associated with the first audio signal source 4 via signal transfer path 20. The circuit labeled 6 is a detector which detects the state of the hook switch 11 of the telephone set and is associated with the second audio signal source 5 via signal transfer path 21. The circuit labeled 7 is a square wave lamp signal source. The circuit 8 is an indicator which indicates the calling state and the hook switch state of the telephone set 2. The relay contacts 9 are operated when a control relay is energized by telephone set 2 being in a called state. Speaker 10 is for the broadcasting of the first and second audio signals. The hook switch contact is labeled 11. Indicator 12 indicates the called state, and diodes D1 and D2 are reverse current prevention diodes.
When the hook switch 11 is down, (i.e., the set is "on-hook") the first audio signal is broadcast from the speaker 10 because current flows from the power source E via the break contact of the hook switch 11 to detector 3 and the first audio signal source is connected with the first audio signal transmission path 20.
When the hook switch 11 is raised and detector 3 becomes deactivated, the broadcast is suspended and indicator 8 is energized from power source E via diode D1 and the hook switch make contact.
When the hook switch 11 of telephone set 2 is down, and the set is called, the relay contacts 9 close and current flows to detector 6 via the make contacts of the relay. After the second audio signal source 5 is connected to the second audio signal transmission path 21, the second audio signal is broadcast from the speaker instead of the first audio signal. Lamp signal source 7 is connected to indicator 8 of the main apparatus via the make contact of the relay and diode D2, and to indicator 12 of the set, and both indicators flash on and off. When the hook switch is raised, detector 6 becomes deactivated, relay 9 is released by its control circuit, broadcasting is suspended, and indicator 12 extinguishes while indicator 8 lights steadily.
Thus, prior art calling circuits required six signal conductors and two power supply conductors to connect station sets with the main apparatus in order to satisfy the several transmission functions of two types of audio signals, detection of the state of the telephone set hook switch, the indication of the state of use of the telephone set, and the lamp signal transmission.
It is therefore a general object of this invention to provide an improved calling circuit for use in a key telephone system.
It is a more specific object of this invention to provide a calling circuit for use with a key telephone system which allows between key telephone station and main apparatus, transmission of two types of audio signals, detection of the state of a telephone set hook switch, and transmission of indicator control signals by means of only two signal and two power supply conductors.